Demand on a higher speed of networks is sharply increased as voice and text oriented services have evolved to video based services due to a rapid expansion of Internet. In addition, achieving a higher capacity network is necessarily required to provide services, which integrate video, data and audio, through one network infrastructure. To meet this demand and requirement, a study on passive optical network (PON) technologies based on optical fiber has actively carried out.
Typical types of the PON technologies are classified as a time division multiplexing passive optical network (TDM-PON) and a WDM-PON. From the two typical types of the PON technologies, a WDM-PON is considered to be an ultimate alternative of a high capacity access network, because it has advantages including a wide bandwidth, a high level of security, easiness of extension, and a high degree of transparency regarding a transmission speed and protocols, etc.
A WDM-PON is a passive optical network which shares an optical fiber by using a method of a wavelength division multiple access (WDMA) and comprises an optical line termination (OLT) being installed at a central office (CO); a remote node (RN) being preferably embodied by an arrayed wavelength grating (AWG); and an optical network termination (ONT) being installed at a subscriber.
FIG. 1 illustrates a structural view of WDM-PON in accordance with prior art. Referring to FIG. 1, RN is installed in a manhole or on an electric pole and is mainly comprised of AWG. However, RN may be embodied by using a wavelength division multiplexer/de-multiplexer (hereinafter referred to WDM), like a thin film filter. RN is connected to a plurality of ONTs, which is equipment for subscribers, by using separate optical fibers.
In a WDM-PON, a guaranteed bandwidth being provided for each subscriber is very broad so that it is possible to integrate services provided by a TV such as an internet protocol television (IP TV), a telephone, a visual telephone, an internet, etc., as can be seen from FIG. 1. FIG. 1 shows an example of a WDM-PON which employs an internet protocol. OLT of a WDM-PON as shown in FIG. 1 requires an L2/L3 switch for separating traffics, depending on services, in order to connect OLT to upper-level equipments (which are specifically equipments being connected to the L2/L3 switch as shown in FIG. 1, such as plain old telephone service (POTS), a stream server, and a core router) depending on respective services.
Each of a plurality of ONTs requires an optical transceiver for communication between OLT and each ONT, while OLT also requires an optical transceiver corresponding to an optical transceiver at anyone of each ONT. AWG, which is a passive device and being installed in RN, performs both a function of de-multiplexing a down-stream signal which is transmitted from OLT and transmitting the de-multiplexed down-stream signal to each ONT and a function of wavelength division multiplexing an up-stream signal which is transmitted from each ONT and transmitting the multiplexed up-stream signal to OLT. Similarly, AWG in OLT also performs both a function of multiplexing a down-stream signal which is transmitted to each ONT and a function of wavelength division de-multiplexing an up-stream signal which is transmitted from each ONT to OLT. Accordingly, the AWG in OLT and the AWG in RN must have the same characteristic. That is, a transmission wavelength in each terminal of the AWG in OLT must be the same as a transmission wavelength in each corresponding terminal of the AWG in RN.
Generally, a TDM-PON may accommodate subscribers distributed in various areas simply by connecting plural optical power splitters and multiple stalling them, if an enough value of power margin in the TDM-PON system is guaranteed. However, in case of a WDM-PON, multiple starring is impossible by using plural optical power splitters only because a specific value of wavelength is assigned to each ONT and a wavelength division multiplexer/de-multiplexer (WDM) must be employed. Thus, an existing WDM-PON is useful where many subscribers, who need a wide bandwidth, are densely distributed in one area, while it is difficult for one WDM-PON to accommodate subscribers who are distributed in a wide area or various areas. Further, even though the number of subscribers to be accommodated by one PON is small, equipments for all subscribers who are available to be accommodated must be installed initially and thus there is a problem that high initial investment costs are required. One method to solve this problem is disclosed in a Korean patent laid-open publication No. 10-2005-0099229, which was filed on Apr. 9, 2004 and was laid-open on Oct. 13, 2005, with the title of “WAVELENGTH DIVISION MULTIPLEXING PASSIVE OPTICAL NETWORK HAVING MULTIPLE BRANCH DISTRIBUTION NETWORK” and a Korean patent laid-open publication No. 10-2005-0092684, which was filed on Aug. 31, 2005 and was laid-open on Sep. 22, 2005, with the title of “MULTIPLE BRANCH WDM-PON SYSTEM USING CYCLIC AWG.” However, in the above referenced laid-open publications, a structure of a filter for combining and splitting up-stream signals and down-stream signals becomes complicated as the number of branches increases. In addition, an insertion loss is also increased due to the use of one or more coupler. Further, it is difficult in terms of maintenance and management because two AWGs (AWG1 and AWG2) having different types of a transmission wavelength characteristic from each other are required to be used in second RNs.